Different sites of an eye fundus of a human being significantly vary in brightness, and for example, a macular area is dark and an optic disc portion is quite bright in comparison to other portions. For this reason, when the eye fundus is presented on a display with brightness at which the macular area is easily observed, an image of the optic disc portion becomes completely white. Conversely, when the eye fundus is presented with brightness at which the optic disc portion is easily observed, an image of the macular area and the like becomes completely black in most cases. This phenomenon has been an obstacle in diagnosis using eye-fundus images for a long period of time. The complete whiteness and blackness described above are attributable to an insufficient dynamic range of an image sensor when an image is photographed and an insufficient dynamic range of a display that presents an eye-fundus image.
As a method for overcoming the insufficient dynamic range of an image sensor, for example, combining images photographed a plurality of times under different exposure conditions has been proposed (Patent Literature 1). However, even though an image with a sufficient dynamic range for expressing an eye fundus is obtained using the method, if the dynamic range of a display which expresses the image is insufficient, the image to be expressed becomes completely white or completely black after all.
As a method for overcoming an insufficient dynamic range of a display, for example, allocating more levels of a grayscale to a target site to be noted than to other portions when the grayscale of an input image is corrected has been proposed (Patent Literature 2). In addition, when there are a plurality of sites to be noted, for example, generating and storing a plurality of images in which the respective sites are easily viewed in a stage in which the images are to be stored has been proposed (Patent Literature 3).